A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
When using an LPP source, the laser radiation itself represents a significant amount of unwanted radiation that is emitted into the EUV lithography tool by scattering and reflection at the plasma. Typically, a CO2 laser having a wavelength of around 10.6 μm is used. Since the optics of the EUV lithographic system have a high reflectivity at 10.6 μm, the laser radiation propagates into the lithography tool with significant power. Part of this power is eventually absorbed by the wafer causing unwanted heating of the wafer.
U.S. Pat. No. 7,196,343 B2 discloses a reflective grazing-incidence spectral purity filter (SPF) for filtering deep ultraviolet (DUV) radiation. This SPF comprises two perpendicular mirrors with an antireflection (AR) coating for DUV radiation of one polarization. By using two perpendicular reflections as shown in FIG. 5, the (mostly unpolarized) DUV radiation is effectively suppressed by absorption in the substrate. Furthermore, the AR coating is characterized in that it has a high reflectivity for EUV, so that EUV radiation is mostly reflected.
This spectral purity filter is not suited for suppressing 10.6 μm radiation in an LPP source for mainly two reasons: AR coatings for 10.6 μm generally have a very low reflectivity for EUV; and conventional mirror substrates reflect rather than absorb 10.6 μm radiation.